1. Field of the Invention
The invention relates to a background calibration method and system, and more particularly, to a background calibration system for calibrating the non-linear distortion of the amplifier applied in an analog-digital converter circuit and method thereof.
2. Description of the Related Art
The amplifier circuit is well known in the art and commonly used in the analog-digital converter (ADC) system, such as the Pipelined ADC or the Sub-ranging ADC. However, the traditional amplifier remains a major challenge in the non-linear distortion phenomena. The non-linear distortion of the amplifier would degrade the performance of ADC system. Therefore, in order to boost the performance of ADC system, the error generated by the non-linear distortion of the amplifier must be compensated. With the progress of semiconductor manufacture process, the design of amplifier is always accompanied by a smaller gain and an increasing non-linear distortion. Up to now, most proposed researches in the art focused on the solution of finite linear gain of the amplifier, and few paid attention to the non-linear distortion problem.
For example, a large group of proposed researches for calibration of non-linear distortion are based on digital calibration themes with “correlation based,” which are not applicable for DC signals. Moreover, some calibration techniques need one more high-accuracy ADC in the circuit to generate a reference signal. The unavoidable ADC would significantly increase the cost and chip area of the circuit. Additionally, some calibration techniques can only perform offline calibrations, which are unable to calibrate for the real-time change of environment or temperature.
Specifically, in the thesis “A 12-bit 75-MS/s pipelined ADC using open-loop residue amplification” by B. Murmann and B. E. Boser, a non-linear calibration method based on the random switch transfer feature and statistic result was proposed. However, this method can merely deal with the signal with normal-distribution pattern, and merely calibrate the third-order non-linear distortion. Above the third-order, this method will not be applicable. Another calibration method with correlation based was proposed by J. P. Keane, P. J. Hurst, and S. H. Lewis in the thesis “Background interstage gain calibration technique for pipelined ADCs.” This method determines the characteristic of non-linear distortion by calculating the output signal of backend ADC with correlation based. Same, this method can only calibrate the third-order non-linear distortion, and it is not capable of calculating the non-linear distortion of DC signal. Still another calibration method was proposed by J. Yuan, N. Farhat and J. V. der Spiegel in the thesis “A 50 MS/s 12-bit CMOS pipeline A/D converter with nonlinear background calibration.” This method utilizes output signals from an additional low-speed and high-accuracy ADC as the reference signals to collect the non-linear distortion signals, and then calculated a proper curve to minimize the distortion error. However, this unavoidable ADC would significantly increase the cost and hardware area of the entire circuit. Additionally, in the thesis “A 12-bit 80-MSample/s pipelined ADC with bootstrapped digital calibration” by C. R. Grace, P. J. Hurst and S. H. Lewis, a foreground calibration method was proposed. This method calculates the coefficient of non-linear distortion before the operation of ADC, and then compensates the error of non-linear distortion according to this coefficient during the operation. As mentioned above, this method can only perform offline calibrations, which are unable to adjust the calibration for the real-time change, such as environmental or temperate difference, or the variety of supply voltages. Lastly, in the thesis “Digital background correction of harmonic distortion in pipelined ADCs” by A. Panigada and I. Galton, a background correction method is proposed. This method utilizes the signals with random sequences to extract the non-linear coefficients with correlation based. The drawback of this method is that the random sequences signals require an additional output signal swing, which increases the loading of circuit under this low supply voltage environment. Moreover, although this method can calibrate high-order non-linear distortion, the requirement of inputting more random sequences would increase more output signal swing, which would dramatically decrease the performance of ADC system.
Therefore, to solve the above-mentioned problems and non-linear limitation of the amplifier, the present invention proposes a novel background calibration system and method for calibrating the non-linear distortion of the amplifier. The present invention calculates the high-order non-linear coefficients by inputting the random sequences in the circuit to calibrate the non-linear distortion of the amplifier. The present invention can dramatically increase the circuit's speed, lower the requirement of amplifier's gain, and reduce the complexity of IC design. Moreover, the accuracy of the non-linear distortion coefficients calculated by the present invention will not be affected by the characteristic signals. Without any extra ADC, the present invention can thereby reduce the implementation cost and power consumption of the amplifier circuit and reduce the precious chip area.